Dark decay controlled xerography



Sept. 12, 1967 c. SNELLING 3,341,326

DARK DECAY CONTROLLED XEROGRAPHY Filed Oct. 1 1962 VOLTAGE HIGH DARK DECAY INVENTOR. CHRISTOPHER SNELlLlNG I A TTORWEV United States Patent ork Filed Oct. 1, 1962, Ser. No. 227,487 Claims. (Cl. 96-15) This invention relates in general to xerography and in particular to an improved xerographic plate.

Xerography, as originally described in US. Patent 2,297,691 to Carlson and later related patents, generally comprises charging a photoconductive insulating member to sensitize it and then subjecting it to a light image or other pattern of activating electromagnetic radiation which serves to dissipate charge in radiation struck area-s, thus leaving a charge pattern or latent electrostatic image conforming to the electromagnetic radiation pattern on the photoconductor. The image is then developed by the deposition of electrostatically attractable finely divided colored material, referred to in the art as toner, on the exposed photoconductor which by virtue of its latent electrostatic image, forms a corresponding toner image on its surface. The toner image may then optionally be viewed in situ or transferred to a copy sheet and fixed.

The photoconductive insulating member, known in the art as a xerographic plate, has undergone continuous improvement and development since the time of the original Carlson patent noted above as is attested to by the following exemplary US. patents, 2,599,542 to Carlson, 2,803,541 to Paris, 2,863,768 to Schalfert, 2,901,348 to Dessauer, and 2,970,906 to Bixby. As might be imagined, very good quality xerographic plates have resulted from this development work. In fact, the faithful rendition and good definition of line copy such as printing, line drawings, and the like, has been so good that copying machines employing these plates have enjoyed wide commercial success within a very short time after their introduction. Although these plates are capable of producing exceptionally good line copy reproductions with relatively simple xerographic apparatus, special techniques or more complex apparatus such as halftone exposure as described in US. Patent 2,598,732 to Walkup and electrode development as described in US. Patent 2,777,418 to Gundlach have in many instances been required to produce really exceptional continuous tone rendition and the complete copying of very large solid dark areas.

It is an object of this invention to provide an improved xerographic plate.

A further object of this invention is to provide an improved method of xerographic copying.

Another object of this invention is to provide an improved xerographic plate capable of reproducing any type of original in conjunction with a very simple type of xerographic copier.

An additional object of this invention is to provide an improved xerographic plate of simple construction.

The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed disclosure of several possible embodiments of the invention, especially when taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a cross-sectional according to this invention.

FIG. 2 is a fragmentary face View of one embodiment of the plate of FIG. 1 with a portion of the photoconductive coating removed to expose the material beneath the photoconductor in the plate.

FIG. 3 is a view of the plate similar to the one in FIG. 2 except that it illustrates a different pattern on the plate interface between the photoconductor and its backing.

view of a xerographic plate FIG. 4 is a voltage versus time graph for two different types of xerographic plate interface materials.

FIG. 5 is an end view of a xerographic plate according to this invention illustrating a charge pattern on the plate and the field lines it produces.

In general, the new xerographic plate according to this invention is fabricated so that it has a fine pattern of areas having different rates of charge loss or decay.

In FIG. 1 there is illustrated a xerographic plate generally designated 11 embodying the concept of this invention. In this sectional view of the plate there is shown a substrate 12 having one type of xerographic plate base properties, overcoated with a thin pattern 13 of a material having a second type of xerographic plate base properties, both underlying a photoconductive insulating layer 14 which may consist of vitreous selenium, cadmium sulfide, or any other photoconductive insulator or layer of photoconductive pigment in an insulating binder known in the art. By way of example, layer 12 may be formed from any material which causes a high rate of charge decay from a charged xerographic plate using it as a base material while layer 13 is formed from a material which causes a relatively low rate of charge decay from a charged xerographic plate using it as a base material when the plate is held in darkness after charging. Some examples of such materials and their theory of operation are discussed hereinafter.

This pattern of materials when used under a photoconductive insulator to form a xerographic plate provides many small areas (corresponding to the pattern) of potential contrast a short time after the plate is uniformly charged. This potential contrast pattern provides the strong fringing fields necessary to good xerographic de velopment as described more fully hereinafter while plate discharging effects from image exposure are superimposed on this built-in potential contrast pattern. It might be said that the image exposure is used to modulate or vary the magnitude of the potential contrast pattern as a whole by discharging, partially or fully, the charge retaining areas.

In one exemplary embodiment of the plate the base layer 12 is made up of a material having suificient electrical conductivity for the charging or sensitization of the xerographic plate and to accommodate the release of electric charge upon exposure of the plate. Desirably, this base member is also sufliciently strong to provide mechanical support of the remainder of the plate so as to make the plate suitable for use in xerographic copying machines. Thus the base member may consist of a metallic plate, web, foil, or the like, or a conductive plastic, glass, paper, or similar member any of which may be in the form of a cylindrical surface, a plane, hexagon, or other shapes. This conductive support member may be relatively rigid as in the case of a metallic plate or cylinder, or may be relatively flexible as with a metallic foil, plastic web, or the like. The term conductive as applied to this backing member 12 is only relative and thus the conductive support must only have high conductivity as compared with the photoconductive insulating layer of the plate. Thus the backing member should have a resistivity lower than about 10 ohms-cm. and preferably lower than about 10 ohm-cm.

Some known xerographic plates suffer from a fairly high rate of potential leakage after charging even in the absence of activating radiation, requiring relatively rapid processing so that the plate is not substantially discharged prior to development. This type of leakage is known in the art as high decay. Plates which hold their charge for relatively long periods of time when not subjected to activating radiation are referred to as having low dark decay."

Dark decay in xerographic plates has largely been controlled by the interposition of a thin insulating layer hereinafter referred to as a barrier layer at the interface of the conductive backing and the photoconductive insulating layers. From a theoretical point of view the operation of a photoconductive insulator on a conductive backing is in itself highly complex and imperfectly understood and with the addition of a barrier layer the situav tion becomes even more complex. While various photoconductive insulating materials may be used, the theory of operation of vitreous selenium on a conductive backing both with and without the barrier layer will be examined and analyzed for purposes of illustration. As partially described above vitreous selenium xerographic plates are generally given an initial uniform positive charge over their surface. This charging is generally accomplished in commercial xerographic machinery by corona discharge from a wire filament or wire filament array spaced slightly from the surface of the plate as described, for example, in US. Patents, 2,588,699" to Carlson, 2,836,725 to Vyverberg, 2,777,957 to Walkup, 2,- 778,946 to Mayo, and others. Other charging techniques such as described in US. Patent 2,833,930 to Walkup known in the 'art and described in the patent literature have also been shown to be feasible and may be employed where desirable. (It should be noted that any one of these charging techniques may be used in charging the novel plate of this invention.) When portions of this charged layer are exposed to a pattern of electromagnetic radiation ofa frequency to which the photoconductive insulating material is sensitive, they are discharged.

The plate described in FIGURES 1-3 is made up of a base 12 selected so that it will cause a high rate of dark decay" from an ordinary charged xerographic plate using it as a base. Some illustrative materials which may be used for this purpose include stainless steel, abraded aluminum, chemically cleaned aluminum, or aluminum subjected to a chromate conversion process including a bath with chromic and phosphoric acids. Layer 13 is made up of a pattern or screen of thin insulating material. This pattern may be in the form of a dot pattern with round, elliptical, square, triangular, or any other regular or irregular dot shape or it may be in the form of a line pattern, or any other broken pattern whether regular, irregular, or random in shape and/or spacing. It should also be noted that instead of making the pattern in the form of dots the outline of a dot pattern may be used for layer 13. Thus layer 13 may comprise an integral layer of insulating materials with a pattern of holes or voids through it. The percentage coverage of the screen or pattern over the base layer 12 may vary very widely without very wide variation in results and even then results vary only in degree. For example, it has been found that plates using screens covering from about 6- to 40% of the total area of the plate base results in fairly low density prints while plates with screen coverage of from about 40 to about 75% coverage of the base plate results in good density prints with very good gray scale reproduction and screens covering from about 75% to about 92% of the plate base result in very high density prints with relatively high contrast. The fineness of the screen or number of dots per linear inch is also an optional plate parameter depending upon the quality of reproduction desired. The number of dots per linear inch on the dot pattern may vary anywhere from 30 to 500 depending upon considerations of quality desired, cost, etc. For purposes of comparison, it should be pointed out that newspaper printing utilizes halftone patterns ranging from about 60 to about 100 lines per linear inch. It has been found that a pattern of approximately 150 dots per linear inch produces an image in which no dots are discernible to the naked eye. The screen material is made of a good" xerographic plate interface material of the type described above and is insulating in nature and may include such diverse materials as aluminum oxide, copper oxide,

oxidized brass, polystyrene, and various organic materials including many of the well known photoresists such as Kodak Photoresist, available from Eastman Kodak Company of Rochester, NY. (believed to be formulated mainly from cinnamic acid esters), and Le Pages printed circuit resist.

High and low dark decay theory and additional (barrier) materials which may be used to fabricate the plate of this invention are described in copending U.S. patent application Ser. No. 707,561 filed Jan. 7, 1958, in the name of Jerry L. Stockdale. Regardless of the theoretical explanation of the operation of insulating or barrier layers in xerographic plates, it has been experimentally found that thin uniform insulating layers on the order of about 2 microns and less at the interface between the photoconductive insulating layer and its conductive backing act to substantially inhibit dark decay in xerographic plates. The effect of thin insulating interfaces in xerographic plates and the tunneling effect in them are more fully described in US. Patent 2,901,348 to Dessauer.

An aluminum oxide pattern may be formed on an aluminum base plate by first forming a uniform layer of aluminum oxide on the surface of the plate by heating or chemical treatment and then scribing or engraving portions of the aluminum oxide off the plate. Materials such as polystyrene may be laid down on plate bases such as clean copper aluminum or stainless steel by mixing them with solvents and printing them on the plate while the photoresist materials referred to may be placed on the plate surface in screen configuration by first laying down a uniform layer of the material and then exposing the layer to a light source through a screen pattern corresponding to the pattern to be provided on the plate. The photoresist is then developed and washed. This removes the unexposed sections of photoresist. The photoconductive insulating layer may then be laid down on the plate surface by vacuum evaporation, casting, or other techniques well known in the art.

The screen pattern may either be very thin in which case it acts according to the tunneling theory described above, or it may be somewhat thicker in which case it acts as an insulator after plate charging both before and after plate exposure. The insulator acts as a tunneling layer at thicknesses below about 2 microns and as an ordinary insulator above about 2 microns. Between charging and exposure a thin or a thick interface layer of insulating screen will have substantially the same elfect on the charge level across a xerographic plate because, as explained above, even a thin layer acts as an insulator and does not allow tunneling until after exposure. Thus either type of insulating layer sets up very many small areas of potential contrast with the charge level on the plate being relatively high over portions of the plate which are directly above the insulating screen material and the charge level on the plate directly above plate backing material which is not covered by portions of the insulating screen being relatively low because of the high dark decay in those areas resulting from charge injection from the conductive plate backing. When an insulating pattern too thick to allow tunneling is used as an interface material, it has the same effect on dark decay as the thinner type insulating layers; however upon exposure it continues to prevent charge injection from the plate backing and thus does not allow charge dissipation through it upon exposure. Although this could not be tolerated with an ordinary xerographic plate having a uniform interface, it may be effectively used in the plates of this invention which have a broken or patterned interface since charge may be injected into the photoconductor from plate areas which are not coated by the insulating pattern, in response to the strong electric field created by the holes formed by illumination of the photoconductor.

The use of a uniform layer of an interface material which inhibits dark .decay in a conventional xerographic plate generally results in a reduction of the original charge placed on the plate of about 1 to 3% in a fixed time Whereas a high dark decay interface material such as abraded or clean aluminum or stainless steel will produce a reduction in the original charge placed on the plate of from 24 to 35% in the same time. These figures are, of course, only illustrative and depend upon the thickness of the photoconductor the like. However, they do illustrate the potential differences which will exist in a plate of the type illustrated in FIGURES 1-3 under certain conditions, a short time after plate charging.

In operating the plate of this invention many techniques generally similar to conventional xerographic copying methods may be followed, with the exception that a short period of time should elapse between charging and development so that the potential contrast may be set up across the plate surface. The discharge effects of image exposure may be applied to the plate at any time prior to development just so that potential contrast and exposure effects are superimposed on the plate. Thus, for example, charging and exposure may take place simultaneously shortly followed by development, or charging may take place just prior to exposure followed shortly by development, etc. The time lapse which passes between charging and development may be controlled by proper selection of materials for the plate. Thus, for example, in fabricating a plate for a continuous automatic copying machine materials are selected so that the dark decay of the two types of areas in the plate are sharply different. This provides the necessary potential contrast pattern on the plate in the relatively short time between charging and development in such a machine. Charging may be carried out with the conventional xerographic charging techniques described above. Development may also utilize any of the known xerographic developing techniques such as cascade as described in US. Patents 2,618,551 and 2,638,416 to Walkup, and powder cloud as described in US. Patent 2,725,304 to Landrigan, among others.

It has often been said in the field of xerography that potential gradients rather than actual potentials are what is developed when thefinely divided colored electroscopic marking material referred to in the art as toner is deposited on a latent electrostatic image on an ordinary xerographic plate. What this statement intends to convey is that in any area of uniform charge (such as a large solid dark area) or low potential gradient such as slowly changing grays in a continuous tone image, on a xerographic plate, the lines of force of the electrostatic fields tend to go in to the plate toward the conductive backing of the plate making these areas difficult to develop with charged toner particles. At the edges of a field or for that matter at any areas of a charge pattern having a high potential gradient (as is the case with line copy originals) a strong fringing field exists. Fringing fields of this type have vertical electrostatic field components extending out above the top surface of the photoconductive layer thus serving to attract toner to these areas during development. This is more clearly shown in FIGURE 5 where there is illustrated a conventional xerographic plate 18 having a grounded conductive backing layer 19 underlying three arbitrarily divided sections of photoconductive insulator 20, 21, and 22. As shown in FIGURE 5 the center area 21 carries a layer of uniform positive charge across its top surface. As is well known and as is shown in section 21 of the photoconductive layer the electrostatic field lines from the charge in the center of area 21 extend almost exclusively to the grounded conductive backing layer 19 of the plate since this is the closest area to this charge which is connected to ground. It should be noted that the lines of force extending from the charge at the edges of the uniformly charged area extend out above the top surface of the photoconductive insulating layer. These fringing field lines extend in curved paths toward sections and 22 of the photoconductive insulating surface which used, initial voltages applied, and

are at a low potential with respect to the charged area 21. It is this principle which accounts for the failure of large solid charged areas to fully develop their centers during the xerographic process when it is carried out with a conventional plate without special techniques or apparatus. The above described phenomena also accounts for a certain lack of discrimination in the gray scale when continuous tone subjects which lack sharp potential gradients are copied on a conventional xerographic plate utilizing conventional techniques and apparatus. With the plate of this invention the broken pattern of high and low dark decay materials effectively acts to modulate the charge pattern left on the plate after uniform charging providing many small adjacent areas of high and low charge each adjacent pair having a high potential gradient and many small fringing fields from one area to the next. Although it has not been determined with certainty how much charge, if any, is retained on the high dark decay areas, it is clear that a potential contrast pattern remains on the plate except in areas which have been exposed to a white original for a relatively long period. With proper exposure of the plate both high and low dark decay areas opposite white or transparent portions of the original will be fully discharged after exposure. In the case of light grays on the original the high dark decay areas of the plate corresponding to the grays of the original will be fully discharged while the low dark decay areas opposite these grays will retain a partial charge increasing in magnitude as the gray darkens. When the grays reach a certain darkness some charge may also be retained on the high dark decay areas after exposure although it is thought that the high decay areas are almost fully discharged but a much larger amount of charge will be retained on the low dark decay areas so that there will still be a substantial contrast in potential between the two immediately adjacent types of areas.

As described above, this exposure-modified contrast pattern on the plate of this invention works very well in conjunction with conventional xerographic developing techniques because the contrast pattern sets up small fringing fields which are well adapted to attract xerographic developers. Since each of these fringing fields emanating from a small discrete portion of the plate produces its own field lines above the plate surface and since each of these small fields is dependent upon the amount of exposure to which it has been subjected, each portion of the plate tends to develop more nearly in conformance with the amount of exposure to which it has been subjected than do conventional xerographic plates whose field lines tend to go down into the plate except when they are used to copy originals which produce high potential gradients on them, thus producing a greatly improved result in the copying of continuous tone and large solid dark areas subjects.

The present invention has been described with reference to certain specific embodiments which have been presented in illustration of the invention. It is to be understood, however, that numerous variations of the invention may be made and that it is intended to encompass such variations within the scope and spirit of the invention as described by the following claims.

What is claimed is:

1. A xerographic plate comprising a continuous photoconductive insulating layer on an electrically conductive base layer and further including a pattern of small, separate, discrete portions of an insulating material on said conductive substrate between portions of said conductive substrate, and portions of said continuous photoconductive insulating layer, said portions of an insulating material being thin relative to said continuous photoconductive insulating layer and having a thickness greater than 2 microns.

2. A xerographic plate comprising a continuous photd conductive insulating layer on an electrically conductive base layer and further including a pattern of small, separate, discrete portions of an insulating material on said Conductive substrate between portions of said conductive substrate, and portions of said photoconductive insulating layer, said portions of an insulating material being thin relative to said continuous photoconductive insulating layer and occurring in the range of from about 40 to about 500 per inch of plate surface.

3. A xerographic plate according to claim 2 in which said photoconductive insulating layer comprises the amorphous form of selenium.

4. A method of copying with a xerographic plate having a continuous surface of a photoconductive insulating material and including a fine pattern of small alternating first and second areas said first areas being characterized by their ability to hold charge for long periods of time in the absence of electromagnetic radiation said second areas being characterized by their relatively rapid rate of charge dissipation with respect to said first areas in the absence of electromagnetic radiation including the steps of applying charge to said xerographic plate, exposing said plate to an original to be copied with electromagnetic radiation actinic to said xerographic plate and developing said xerographic plate with finely divided electroscopic marking particles said charging and developing steps being temporally separated so as to allow for the formation of a fine potential contrast pattern across the surface of said plate prior to development.

5. A method according to claim 4 in which the charging and exposure steps are carried out simultaneously.

6. A method of xerographic copying comprising charging a xerographic plate made up of a continuous photoconductive insulating layer on a substrate composed of a pattern of alternating small areas of first and second materials, said first material being characterized by its ability to inject charge carriers into said photoconductive insulating layer when itis charged prior to its illumination with activating electromagnetic radiation, said second material being characterized byits ability to prevent the injection of charge carriers into said photoconductive insulating layer after its charging and prior to its illumination with activating electromagnetic radiation, exposing said xerographic plate to an original to be copied with activating electromagnetic radiation and developing said xerographic plate with finely divided, electroscopic, mark ing particles, said charging and developing steps being temporally separated to allow for the formation of a fine potential contrast pattern in unexposed areas of said plate prior to its development.

7. A method according to claim 6 including carrying out said charging and exposing steps simultaneously with both being separated in time from said development step.

8. A method of xerographic reproduction comprising charging a xerographic plate made up of an electrically conductive substrate with a thin insulating barrier layer covering alternate, small, adjacent areas of said substrate and a continuous photoconductive insulating layer on the top surface of said xerographic plate, exposing said xerographic plate to an original to be copied with activating electromagnetic radiation and developing said xerographic plate with finely divided, electroscopic, marking particles, said charging and developing steps being separated in time so as to allow for the formation of a fine potential contrast pattern across the surface of said plate in unexposed areas prior to development.

9. A method of xerographic reproduction comprising charging a xerographic plate made up of an electrically conductive substrate with a thin insulating barrier layer covering alternate, small, adjacent areas of said substrate and a continuous amorphous selenium layer on the top surface of said xerographic plate, exposing said xerographic plate to an original to be copied with activating electromagnetic radiation and developing said xerographic plate with finely divided, electroscopic, marking particles, said charging and developing steps being separated in time so as to allow for the formation of a fine potential contrast pattern across the surface of said plate in unexposed areas prior to development.

10. A method of xerographic copying comprising charging a xerographic plate having a continuous surface of a photoconductive insulating material overlying a substrate made upof an electrically conductive material with a pattern of small, separated, discrete areas coated with a thin layer of an electrical insulator occurring on said electrically conductive layer in a range of from about 40 to about 500 per inch of plate surface, exposing said plate to an original to be copied with activating electromagnetic radiation and developing said plate with finely divided, electroscopic, marking particles, said charging and developing steps being separated in time so as to allow for the formation of a fine potential contrast pattern across the surface of said plate in unexposed areas prior to the development.

References Cited UNITED STATES PATENTS 2,543,051 2/1951 Oughton et a1 96-1 2,599,542 6/1952 Carlson 96-1 2,844,543 7/1958 Fotland 96-1 2,853,383 9/1958 Keck 96-1 2,881,340 4/1959 Rose 252-501 2,901,348 8/1959 Dessauer et al. 96-1 2,917,385 12/1959 Byrne 96-1 3,005,707 10/1961 Kallmann et 'al. 96-1 3,031,572 4/1962 Ryan 96-1 3,234,017 2/1966 Heyl et al 96-1 3,281,240 10/ 1966 Cassiers et al. 96-1 NORMAN G. TORCHIN, Primary Examiner. C. E. VAN HORN, Assistant Examiner. 

1. A XEROGRAPHIC PLATE COMPRISING A CONTINUOUS PHOTOCONDUCTIVE INSULATING LAYER ON AN ELECTRICALLY CONDUCTIVE BASE LAYER AND FURTHER INCLUDING A PATTER OF SMALL, SEPARATE, DISCRETE PORTIONS OF AN INSULATING MATERIAL ON SAID CONDUCTIVE SUBSTRATE BETWEEN PORTIONS OF SAID CONDUCTIVE SUBSTRATE, AND PORTIONS OF SAID CONTINUOUS PHOTOCONDUCTIVE INSULATING LAYER, SAID PORTIONS OF AN INSULATING MATERIAL BEING THIN RELATIVE TO SAID CONTI NUOUS PHOTOCONDUCTIVE INSULATING LAYER AND HAVING A THICKNESS GREATER THAN 2 MICRONS. 